Inorganic photoconductive materials recently in use include amorphous silicon, amorphous selenium, cadmium sulfide, zinc oxide, and the like. However, some of these materials are expensive because of difficulties in production thereof, while others are toxic and disadvantageous from the standpoint of environmental protection.
On the other hand, as organic photoconductors, ones of the type comprising, in particular, a charge-generating material and a charge-transporting material which respectively perform their functions are proposed extensively (e.g., U.S. Pat. No. 3,791,826). In this type, there is the possibility that a high-sensitivity electrophotographic photoreceptor might be obtained by using a substance which efficiently generates carriers (The term "carriers" means "charges"; the same applies hereinafter) as the charge-generating material in combination with a substance having high carrier-transporting ability as the charge-transporting material.
Of these materials, the charge-transporting material is required to efficiently receive the carriers generated in the charge-generating material upon illumination in an electric field and permit them to rapidly move through the photosensitive layer to extinguish the surface charges promptly. The speed at which carriers move per unit electric field is called carrier mobility. A high carrier mobility means that carriers rapidly move in the charge-transporting layer. Any charge-transporting substance has its intrinsic carrier mobility and, hence, it is necessary that for attaining a high carrier mobility, a material having a high carrier mobility be employed. However, the attainable carrier mobilities have not yet reached a sufficient level.
Further, in the case of applying a charge-transporting substance after dissolving it in an organic solvent along with a binder polymer, it is necessary to form a thin homogeneous organic coating film free from crystallization and pinhole formation. This is because when a high electric field is applied to the thin film obtained, the part having microcrystals or pinholes undergoes dielectric breakdown or causes noise.
In addition to the satisfactory properties of the charge-generating substance and of the charge-transporting substance, it is also important that carriers should be efficiently injected from the charge-generating substance into the charge-transporting substance, i.e., from the charge-generating layer into the charge-transporting layer. This injection of charges depends on the properties of the interface between the charge-generating substance (or charge-generating layer) and the charge-transporting substance (or charge-transporting layer) and varies with combinations of various substances. Since a charge-transporting material should meet various requirements as described above, charge-transporting substances having a variety of properties are being developed.
Among conventional charge-transporting materials, the styryl compound represented by the formula ##STR3## is, for example, proposed in JP-A-60-174749. (The term "JP-A" as used herein means an "unexamined published Japanese patent application.")
Moreover, a styryl compound represented by the following general formula (2) ##STR4## (wherein R.sup.1' represents an optionally substituted alkyl group or an optionally substituted aryl group, R.sup.2' represents a hydrogen atom, an optionally substituted alkyl group, or an optionally substituted aryl group, and Ar represents an optionally substituted aryl group) is proposed in JP-A-60-175052.
Furthermore, compounds similar to the compound (2) above are proposed in, for example, JP-A-62-120346, JP-A-1-217357, JP-A-4-57056, and JP-A-4-292663.
The demand for charge-transporting materials is growing more and more, with which there is a desire for a newer material which is capable of satisfying various requirements.
In JP-A-4-57056, for example, there is a description to the effect that the compound K specified below partly separated out as crystals during the preparation of a photoreceptor because of the poor solubility of the compound. ##STR5##
There has hence been a desire for the development of a new material which not only is satisfactory in such solubility and other properties but also is capable of attaining a high carrier mobility.